Replicating an Actor will make a copy of that Actor on all Clients but the only stuff replicated on it is that which is marked for replication. So in any of your derived classes, this may be nothing. However, the base class and many Actor classes all have variables that are replicated. The Actor, for instance, has a variable called Owner that is replicated and another called Instigator. If you set bReplicateMovement to true, it will also replicate things like location, rotation, velocity, and if it has a component simulating physics, it will replicate AngularVelocity as well.
So basically, setting an Actor as replicated will only replicate things marked as replicated, which there are several things in the Actor.h you can see have been set to replicate.